System and Method for User Equipment Cooperation

ABSTRACT

An embodiment method includes receiving, by a first user equipment (UE), a message, for a second UE, transmitted over a plurality of resource blocks (RBs) on behalf of a communications controller and determining a plurality of log-likelihood ratios (LLRs) in accordance with the received plurality of RBs. The method also includes transmitting, a subset of the determined LLRs to the second UE.

This application is a continuation of U.S. patent application Ser. No.14/860,076, filed on Sep. 21, 2015, entitled “System and Method for UserEquipment Cooperation,” which is a continuation of U.S. patentapplication Ser. No. 14/027,919, filed on Sep. 16, 2013, now U.S. Pat.No. 9,148,253 issued on Sep. 29, 2015, entitled “System and Method forUser Equipment Cooperation,” which applications are hereby incorporatedherein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a system and method for wirelesscommunications, and, in particular, to a system and method for userequipment (UE) cooperation.

BACKGROUND

In wireless networks, the rate of data consumption and the density ofmobile devices are increasing. With the increase in the density ofmobile devices, device-to-device (D2D) communications via localconnectivity has emerged. D2D communications are often performed overWiFi or Bluetooth over an unlicensed band. However, D2D may also beperformed over a licensed cellular band in a cellular network viacentral controlled dedicated bands with an improved interferencecontrolled environment.

D2D communications may be used for user equipment (UE) cooperation,which may increase cellular throughput and coverage. In a cellularsystem, there are significantly more idle UEs than serving UEs at agiven time. When UEs are close to each other, UE cooperation may offerthe benefits of virtual multiple-input multiple-output (MIMO) receivers.

SUMMARY

Technical advantages are generally achieved, by embodiments of thisdisclosure which describe systems and methods for user equipment (UE)cooperation.

An embodiment method includes receiving, by a first user equipment (UE),a message, for a second UE, transmitted over a plurality of resourceblocks (RBs) on behalf of a communications controller and determining aplurality of log-likelihood ratios (LLRs) in accordance with thereceived plurality of RBs. The method also includes transmitting, asubset of the determined LLRs to the second UE.

An embodiment method includes receiving, by a central controller from afirst UE, a first signal-to-noise ratio (SNR) measurement correspondingto a resource block (RB) and receiving, by the central controller from asecond UE, a second SNR measurement corresponding to the RB. The methodalso includes determining a selected UE to transmit a set oflog-likelihood ratios (LLRs) corresponding to the RB in accordance withthe first SNR measurement and the second SNR measurement.

An embodiment central controller includes a processor and anon-transitory computer readable storage medium storing programming forexecution by the processor. The programming includes instructions toreceive, from a first UE, a first signal-to-noise ratio (SNR)measurement corresponding to a first resource block (RB) and receive, bythe central controller from a second cooperating UE, a second SNRmeasurement corresponding to the first RB. The programming also includesinstructions to determine a selected UE to transmit a set oflog-likelihood ratios (LLRs) corresponding to the first RB in accordancewith the first SNR measurement and the second SNR measurement.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates an embodiment system for user equipment (UE)cooperation;

FIG. 2 illustrates a diagram for an embodiment method of log-likelihoodratio (LLR) combining;

FIG. 3 illustrates a diagram of physical resource mapping;

FIG. 4 illustrates a flowchart for an embodiment method of UEcooperation performed by a communications controller;

FIG. 5 illustrates a flowchart for an embodiment method of UEcooperation performed by a cooperating UE;

FIG. 6 illustrates a flowchart for an embodiment method UE cooperationby a target UE;

FIG. 7 illustrates a flowchart for an embodiment method of selectingLLRs to forward by a cooperating UE;

FIG. 8 illustrates a flowchart for an embodiment method of selectingLLRs for forwarding to a target UE in LLR combining;

FIG. 9 illustrates a flowchart for another embodiment method ofselecting LLRs for forwarding to a target UE in LLR combining;

FIG. 10 illustrates a flowchart for an additional embodiment method ofselecting LLRs for forwarding to a target UE in LLR combining;

FIG. 11 illustrates a flowchart for another embodiment method ofselecting LLRs to forward to a target UE in LLR combining; and

FIG. 12 illustrates a block diagram of an embodiment general-purposecomputer system.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

One user equipment (UE) cooperation technique is for cooperating UEs toforward log-likelihood ratios (LLRs) of decoded signals to target UEs.However, this approach typically requires extensive bandwidth, becauseeach cooperating UE transmits one LLR for each encoded bit. Because ofthe high bandwidth, another method of UE cooperation may be desired.

Aspects of this disclosure provide UE cooperation techniques thatexploit the forwarding of selected LLRs by cooperating UEs to a targetUE. Notably, LLRs for forwarding may be selected so the most useful LLRsare forwarded and less useful LLRs are not forwarded.

FIG. 1 illustrates system 100 for UE cooperation. System 100 comprisescommunications controller (CC) 102 and a plurality of UEs. Four UEs aredepicted, but fewer or more may be present. Communications controller102 may be any component capable of providing wireless access by, interalia, establishing uplink and/or downlink connections with the UEs, suchas a base station, an enhanced base station (eNB), a picocell, afemtocell, and other wirelessly enabled devices. The UEs may be anycomponent capable of establishing a wireless connection withcommunications controller 102, such as cell phones, smart phones,tablets, sensors, etc. The UEs include cooperating UEs (CUEs) 106, 108,and 112, and target UE (TUE) 110 in neighborhood 104. When acommunication starts, communications controller 102 broadcasts data tothe UEs. In one example, communications controller 102 is aware of thecooperation amongst the UEs. In another example, communicationscontroller 102 is not aware of the UE cooperation. Cooperating UEs 106,108, and 112 and target UE 110 listen to the data broadcast bycommunications controller 102. Then, some or all of cooperating UEs 106,108, and 112 may forward data to target UE 110 to assist target UE 110.The target UE may discover and select a group of nearby cooperating UEs,such as cooperating UEs 106, 108, and 112.

One example UE cooperation strategy is decode and forward (D&F). In D&F,each cooperating UE attempts to decode the information received from thecommunications controller destined for the target UE. The successfullydecoded information is re-encoded and forwarded to the target UE fromthe cooperating UEs. The target UE decodes the information received fromthe communications controller and the relayed information received fromthe cooperating UEs. If none of the UEs can decode the information, thetransmission fails. D&F is well suited for the situation when the linksbetween the communications controller and the cooperating UEs issignificantly better than the link from the communications controller tothe target UE. However, at least one of the cooperating UEs or thetarget UE has to be able to decode the information for D&F to beeffective. Thus, D&F has a performance worse than virtual multiple-inputmultiple-output (VMIMO) even with perfect device-to-device (D2D) links.

Another UE cooperation strategy is amplify and forward (A&F). In A&F,the cooperating UEs amplify the analog noisy signal received from thecommunications controller to its own transmitter power and forward theamplified noisy signals to the target UE.

In another example of UE cooperation, compress and forward (C&F) isperformed. The cooperating UEs compress and quantize the signalsreceived from the communications controller. Then, the cooperating UEsre-encode the quantized bits for forwarding to the target UE. Thecooperating UEs also forward their access link quality to the target UE,so the target UE may perform joint reception decoding. The signaltransmitted is:

x _(c,i) =y _(c,i) +w _(j),

where y_(c,i) is received signal at the ith cooperating UE, and w_(j) isthe quantization or compression noise. The power distribution of thecompression noise depends on the method of the quantization andcompression.

If the D2D links are assumed to be ideal, the target UE may perform ajoint reception (JR) strategy. In JR, the received signal at thecooperating UE is:

y _(c,i) =h _(a,i) x _(s) +n _(a,i),

where h_(a,i) is the instantaneous access link channel gain for the ithcooperating UE, x_(s) is the transmitted signal from the communicationcontroller, and n_(a,i) is the additive white Gaussian noise (AWGN) atthe access link for the ith cooperating UE. The received signal at thetarget UE is:

y _(d) =h _(d) x _(s) +n _(d),

where h_(d) is the instantaneous access link channel gain in the channelfrom the cooperating UE to the target UE and n_(d) is the AWGN for thechannel from the cooperating UE to the target UE. It may be assumed thath_(a,i) and y_(c,i) are known by the target UE.

To decode the received information, the target UE applies amultiple-input multiple-output (MIMO) receiver, such as a MIMO minimummean squared error (MMSE) receiver. The performance of JR is similar tothat of VMIMO, and provides an upper bound to the performance achievedby UE cooperation. In the single antenna case, the receiver is amaximal-ratio combing (MRC) receiver.

C&F is especially useful when the cooperating UEs are located near thetarget UE. One type of C&F scheme is LLR combining. The LLR, the ratioof two probabilities that indicates the reliability of bits, is givenby:

${{LLR} = {\log \frac{\Pr \left( {{x = \left. 1 \middle| y \right.},h} \right)}{\Pr \left( {{x = \left. 0 \middle| y \right.},h} \right)}}},$

where y is the received signal, h is the channel gain, x is the binaryinformation bit or encoded bit. LLR combining is a soft combining basedUE cooperation strategy. Instead of compressing the received signals,the cooperating UEs demodulate and compute the LLRs of the encoded bits.Then, the cooperating UEs quantize the LLRs, and re-encode them forforwarding to the target UE. The target UE recovers the LLRs from thecooperating UEs by decoding the quantized LLR bits, and combines thereceived LLRs from the cooperating UEs and the communicationscontroller. The combined LLR is the soft input for a turbo decoder,which then decodes the information.

FIG. 2 illustrates diagram 120 for LLR combining. The communicationscontroller receives information bits for transmission to a target UE.The received information bits are encoded using a turbo encoder. Afterencoding, the information is modulated and mapped to resources. Thecommunications controller then broadcasts the information on accesslinks to cooperating UEs and the target UE.

The cooperating UEs receive and demodulate the information. Next, thecooperating UEs quantize the information, and transmit it on D2D linksto the target UE.

The target UE receives the information on a direct access link from thecommunications controller. Then, the target UE demodulates and computesthe LLRs for the direct access information. Also, the target UE receivesthe quantized information from the cooperating UEs and recovers the LLRsfrom the cooperating UEs. The target UE adds computed and recovered LLRstogether. Then, the combined LLRs are used by the turbo decoder todecode the information.

When there are multiple cooperating UEs, the amount of bandwidth andtime resources for LLR combining are significant. Frequency selectivityof the access links may be exploited to reduce the bandwidth. The LLRsrepresent the reliability of the encoded bits. Because of the variationof channel qualities in different bands, the reliabilities of thereceived encoded bits are different for each UE. Thus, only good orreliable LLRs may be forwarded to the target UE to reduce the burden onthe D2D links.

The variation in reliability of LLRs depends on the fading channels andthe RB allocation. FIG. 3 illustrates RB distribution 130 for a longterm evolution (LTE) system in time and frequency. One transmission timeinterval (TTI) is pictured. For each transport block, the physical layerresource can be distributed to achieve the maximum frequency diversity.RBs 132 are selected for frequency diversity. Alternatively, continuousresource block assignment may be used.

For binary phase-shift keying (BPSK), the received signal for a singleencoded bit is given by:

y=h√{square root over (E _(s))}b+n,

where b=1 when x=0 and b=−1 when x=1, and n is the Gaussian noise, E_(s)is the symbol energy. The LLR is defined as the log of the ratio of theprobabilities of input bits when observing output y as the receivedsignal, or:

${LLR} = {\log {\frac{\Pr \left( {{x = \left. 1 \middle| y \right.},h} \right)}{\Pr \left( {{x = \left. 0 \middle| y \right.},h} \right)}.}}$

The LLR may be computed as:

${{LLR}\left( x \middle| y \right)} = {{\log \left( \frac{p\left( {\left. y \middle| x \right. = 1} \right)}{p\left( {\left. y \middle| x \right. = 0} \right)} \right)} + {{\log \left( \frac{P\left( {x = 1} \right)}{P\left( {x = 0} \right)} \right)}.}}$

The prior LLR is:

${{LLR}(x)} = {{\log \left( \frac{P\left( {x = 1} \right)}{P\left( {x = 0} \right)} \right)}.}$

For equal prior probabilities, LLR(x)=0.

Because the noise follows a Gaussian distribution,

${{p\left( {\left. y \middle| x \right. = 1} \right)} = {\frac{1}{\sqrt{2{\pi\sigma}_{n}^{2}}}e^{- \frac{{{y + {h\sqrt{E_{s}}}}}^{2}}{2\sigma_{n}^{2}}}}},{and}$${{p\left( {\left. y \middle| x \right. = 0} \right)} = {\frac{1}{\sqrt{2{\pi\sigma}_{n}^{2}}}{e^{- \frac{{{y + {h\sqrt{E_{s}}}}}^{2}}{2\sigma_{n}^{2}}}.{Then}}}},{{\log \left( \frac{P\left( {\left. y \middle| x \right. = 1} \right)}{P\left( {\left. y \middle| x \right. = 0} \right)} \right)} = {{- 4}{Re}\left\{ {h^{*}y} \right\} \frac{\sqrt{E_{s}}}{N_{o}}}},$

where h is the instantaneous channel gain, Re{h*y} is the function oftaking the real value of a complex variable.

When N cooperating UEs are available and access links with the sameaverage signal-to-noise ratio (SNR), the post-combining LLR that seversas the soft input of the turbo decoder for a BPSK or QPSK modulation is:

${{LLR}_{combined} = {\sum\limits_{i}^{N + 1}{4{Re}\left\{ {h_{i}^{*}y_{i}} \right\} \frac{\sqrt{E_{s}}}{N_{o}}}}},$

where i is the index of cooperating UEs. For BPSK and QPSK modulation,LLR combining is equivalent to MRC combining for single receiveantennas. Therefore, LLR combining may achieve the same performance asthe JR upper bound. However, for higher level modulation, such as 16QAM,the results are not equivalent to MRC, and LLR combining performsslightly worse than the JR upper bound. However, LLR combining is morepractical than JR, because it does not forward channel informationseparately.

Because the D2D resources are limited, the number of bits required toreliably forward the LLRs to the target UE may be reduced byquantization and compression. LLRs may be quantized in a variety ofways. In one example, the correlation between the received signalsacross different UEs is considered, and compression and decoding isjointly performed. In another example, each UE quantizes the LLRsseparately, and a fixed number of bits are used to quantize each LLRvalue. Therefore, the number of quantized bits used to forwardreliability over the D2D links is proportional to the amount of LLRsthat are forwarded from the cooperating UEs to the target UE.

The quantization and compression process is a mapping function Q(·),which maps the a LLR value to a sequence of binary information bits, as:

{right arrow over (x)}=(

,

, . . . ,

=Q(LLR_(CUE) ₁ ,LLR_(CUE) ₂ , . . . ,LLR_(CUE) _(N) ),

where q represents the total number of bits used to represent all theLLR information of the cooperating UEs after quantization andcompression. Thus, the design objective of the quantizer may be:

_(Q) ^(Min) U(LLR_(CUE) ₁ ,LLR_(CUE) ₂ , . . . ,LLR_(CUE) _(N),LLR_(TUE) ,{circumflex over (x)}),

subject to:

q≤c.

U(·) is the utility function or the cost to minimize and c is themaximum number of bits that can be reliably forwarded to the target UEdue to the limitation of D2D resources. The cost function is a functionof the analog value of the cooperating UE LLRs and the quantizationoutput. Also, the cost function may depend on the LLR values of thetarget UE. If the target UE already has a good channel quality, thequantized LLRs from the cooperating UEs proved less benefit. In oneexample, the cost function is a block error rate (BLER).

In practice, it is problematic to find a simple expression of the costfunction. If the cost function is given, the optimal quantization andcompression algorithm may be problematic to implement. A scalablequantizer with a fixed number of bits to represent the LLR values may beused.

An example quantizer is based on Lloyd-Max quantization design. For afixed level of quantization, a set of threshold values andrepresentative values are found to minimize the average mean squarederror. The quantization is based on:

{tilde over (x)} _(i) =Q(LLR_(i))=t _(j),

if

a _(j−1)<LLR_(i) ≤a _(j)(j=1,2, . . . m),

where m is the number of quantization levels. For a k-bit per LLRquantizer,

m=2^(k),

and the boundary of the quantizer is given by:

a _(j)(j=0,1, . . . ,m),

where

a ₀=−∞, and

a _(m)=+∞.

The representative values of the quantizer output are:

t _(j)(j=1,2, . . . ,m).

The objective of a Lloyd-Max scalar quantizer design is to find theoptimal boundaries and representative values to minimize the meansquared error. That is,

_(a,t) ^(Min) E(LLR,{right arrow over (x)})=∫_(−∞) ^(+∞) f_(LLR)(x)(x−{tilde over (x)})² dx=Σ _(i=1) ^(n)(LLR_(i) −{tilde over(x)} _(i))².

The optimal solution should satisfy:

a _(j)=½(t _(j) +t _(j+1))j=1,2, . . . ,m=1, and

t _(j) =E(LLR|a_(j−1)≤LLR≤a _(j))=∫_(a) _(j−1) ^(a) ^(j) xf _(LLR)(x)dx.

Thus, the boundaries and representative values may be iterativelyobtained.

An embodiment method of UE cooperation using LLR combining isillustrated by flowchart 140 in FIG. 4 performed by a communicationscontroller, flowchart 160 in FIG. 5 performed by a cooperating UE, andflowchart 180 in FIG. 6 performed by a target UE. Initially, inflowchart 140, the communications controller assembles information bitsfor transmission to the target UE in step 142.

Next, in step 144, the communications controller performs turbo encodingon the assembled information bits. This is to achieve reliableinformation transfer in the presence of data-corrupting noise usingforward error correction (FEC) codes.

Then, in step 146, the communication controller modulates theturbo-encoded information. A variety of modulation techniques may beused, including phase-shift keying (PSK), frequency-shift keying (FSK),amplitude-shift keying (ASK), and quadrature amplitude modulation (QAM).For example, BPSK or QPSK may be used.

In step 148, the communications controller maps RBs for transmission infrequency and time. In one example, the RBs are distributed as in PRBdistribution 130 for maximum frequency diversity. In another example,RBs are continuously assigned.

The communications controller broadcasts the information in step 150.The information is destined for the target UE, and also broadcast to thecooperating UEs.

FIG. 5 illustrates flowchart 160 for a method of LLR combining by acooperating UE. This method may be performed by multiple cooperating UEsat a time. Initially, in step 162, the cooperating UE receives data fromthe communications controller. The received data is destined for thetarget UE, not the cooperating UE.

Then, in step 164, the cooperating UE demodulates the received data. Thedemodulation may use PSK, FSK, ASK, or QAM for demodulation. Forexample, demodulation may be performed using BPSK or QPSK.

Next, in step 166, the LLRs of the demodulated data are calculated. AnLLR, the ratio of the probability of a bit being a one to theprobability of the bit being a zero, indicates the reliability of thatbit. For example, the LLR for BPSK modulated signals may be calculatedas:

${\log \left( \frac{P\left( {\left. y \middle| x \right. = 1} \right)}{P\left( {\left. y \middle| x \right. = 0} \right)} \right)} = {{- 4}{Re}\left\{ {h^{*}y} \right\} {\frac{\sqrt{E_{s}}}{N_{o}}.}}$

In step 168, LLRs for transmission to the target UE are selected by thecooperating UE. In one example, the LLRs for transmission are selectedacross different RBs of the same cooperating UE by the cooperating UE.In another example, the LLRs for transmission are selected by a centralcontroller for multiple cooperating UEs. Alternatively, the LLRs may beselected through a hybrid approach by comparing the SNRs for bothdifferent RBs and different cooperating UEs.

The selected LLRs are quantized by the cooperating UE in step 170. Forexample, a fixed number of bits per LLR may be used for quantization.

In step 172, the quantized LLRs are transmitted by the cooperating UE tothe target UE.

FIG. 6 illustrates flowchart 180 for a method of performing UEcooperation by a target UE. In step 182, the target UE receives datafrom the communications controller.

Then, in step 184, the target UE demodulates the transmission from thecommunications controller. The demodulation may use PSK, FSK, ASK, orQAM for demodulation. For example, BPSK or QPSK may be used.

Next, in step 186, the target UE computes the LLRs for the demodulatedsignals. The LLRs are:

${\log \left( \frac{P\left( {\left. y \middle| x \right. = 1} \right)}{P\left( {\left. y \middle| x \right. = 0} \right)} \right)}.$

In step 198, the target UE optionally coordinates with cooperating UEs.The target UE may broadcast a SNR threshold and/or an index of RBsindicating an SNR that is below the threshold. Also, the target UE mayreceive an indication of which RBs will be broadcast by the cooperatingUE.

The target UE receives LLRs from one or more cooperating UEs in step188.

In step 194, the LLRs computed from the transmission from thecommunications controller are added to the LLRs received from thecooperating UEs.

Finally, in step 196, turbo decoding is performed on the LLRs. The turbodecoding uses FEC codes.

In LLR combining, the LLRs to be forwarded may be selected based onfrequency selective scheduling (FSS), so only a portion of the LLRs areforwarded. RBs may be the minimum unit for LLR selection. Thus,cooperating UEs only forward LLRs of certain RBs to the target UE. TheLLRs for forwarding may be selected based on comparing the SNRs ofdifferent RBs for different cooperating UEs in different dimensions. TheRBs may be compared horizontally across different RBs for the samecooperating UE, vertically for the same RB across different UEs, or bycomparing the SNRs for both different RB s and different UEs.

FIG. 7 illustrates flowchart 260 for a method of forwarding LLRs by acooperating UE. Initially, in step 262, the SNRs of the RBs aredetermined. The average SNR for each RB may be computed using theeffective exponential SNR mapping (EESM) average of sampled SNRs atdifferent tones of the RBs.

1 Then, in step 264, the cooperating UE determines the LLRs of which RBsto transmit based on a utility or cost function. This may be performedwith the assistance of a centralized controller. The utility functionmay be defined as a cost function of the SNRs for the cooperating UEsand the target UE for the assigned RBs. D2D link qualities may serve asadditional parameters when they are known by the centralized controller.The resource usage on the D2D links may be constrained as the totalamount of LLRs forwarded. The decision of whether a particularcooperating UE forwards the LLRs of a particular RB involves minimizingthe utility function subject to the resource constraint. Thus:

${\underset{D_{i,j}{({{i = 1},2,\; \ldots \;,{N;{j = 1}},2,\; \ldots \;,M})}}{Min}{f\left( {{\gamma_{i,j}\left( {{i = 1},2,\ldots \mspace{14mu},{N;{j = 1}},2,{\ldots \mspace{14mu} M}} \right)},{\gamma_{j}^{\prime}\left( {{j = 1},2,\ldots \mspace{14mu},M} \right)}} \right)}},$

subject to:

Σ_(i=1) ^(N)Σ_(j=1) ^(M) D _(i,j) ≤T,

where N is the total number of CUEs, M is the total number of assignedRBs, i and j are the indexes of CUEs and RBs, respectively; γ and γ′ arethe SNRs of CUE and TUE, respectively. AndD_(i,j)∈{0,1} is the decisionvariable which represents whether LLRs corresponds to j-th RB of i-thCUE are selected to transmit.

In an embodiment method of LLR combining, the cooperating UEs selectLLRs for forwarding. This method has a low signaling overhead, with theD2D bandwidth reduced by forwarding a small number of LLRs from thecooperating UEs to the target UE. This method does not require acentralized controller. The cooperating UEs make decisions based ontheir own SNRs with minimal signaling overhead.

FIG. 8 illustrates flowchart 200 for a method of selecting LLRs fortransmission by a cooperating UE in LLR combining. This method may beperformed by multiple cooperating UEs at a time. Initially, in step 202,the cooperating UE determines a threshold. In one example, threshold isa predefined threshold. In another example, the threshold is determinedbased on a transmission from the target UE. In an additional example,the threshold is determined by the central controller. The threshold maybe between 0 dB and to dB, for example 5 dB.

Optionally, in step 204, the cooperating UE receives a transmission fromthe target UE indicating which RBs the target UE needs help with. Forexample, the cooperating UE receives an index of the RBs of the targetUE that are below the threshold from the target UE. In an example, theindex of RBs is represented by one bit for each RB.

Next, in step 206, the cooperating UE determines the SNRs of the RBs ithas received from the communications controller. The SNR compares theratio of the RB signal to the level of the background noise.

Then, in step 208, the cooperating UE determines for which RBs tobroadcast the corresponding LLRs. The cooperating UE may decide totransmit the LLRs of RBs when the index of RBs indicates that the targetUE needs help and the SNRs of the RBs are above the threshold. When theSNR of an RB at the target UE is high, the cooperating UE does not needto transmit the corresponding LLR, because the target UE already has agood signal. When the SNR of the RB at the cooperating UE is low,transmitting the corresponding LLRs would not be very helpful. However,when the SNR of the RB at the target UE is low and the SNR of the RB atthe cooperating UE is high, transmitting the corresponding LLRs addssignificant value.

In another example, in step 208, the best M out of N total RBs in termsof SNR at the cooperating UE are transmit to the target UE. This reducesthe D2D resource usage to M/N of the usage when forwarding all the LLRs.In an additional example, M/N RBs are transmitted only when the SNR ofRB at the target UE is less than a threshold.

Finally, in step 210, the cooperating UE transmits a message to thetarget UE indicating which RBs for which the cooperating UE willtransmit the corresponding LLRs.

FIG. 9 illustrates flowchart 290 for a method of selecting LLRs fortransmission by cooperating UEs to a target UE, performed by the targetUE. Initially, in step 292, the target UE determines a threshold. In oneexample, the threshold is predetermined. In another example, the targetUE selects the threshold. The target UE may dynamically determine thethreshold based on the modulation and coding scheme (MCS), the SNR ofits own access link, and the available D2D resources. In an additionalexample, the threshold is determined by the central controller andtransmitted from central controller to the target UE.

After the target UE selects the threshold, it broadcasts the selectedthreshold to the cooperating UEs in step 294. When the threshold ispredetermined, the target UE skips steps 292 and 294, and proceedsdirectly to step 296.

In step 296, the target UE determines the SNRs of the RBs. The SNRindicates the ratio of the RB signal power to the background noise. Theaverage SNR for each RB may be computed using the EESM average ofsampled SNRs at different tones of the RB.

Then, in step 298, the target UE compares the SNRs of the RBs to thethreshold. The target UE determines an RB index that indicates which RBshave SNRs below the threshold. For example, the RB index may have onebit per RB.

Next, in step 300, the target UE broadcasts the RB index to thecooperating UEs.

In response, the target UE receives messages from the cooperating UEsindicating which RBs which cooperating UEs will transmit thecorresponding LLRs for in step 302.

In another example method for selecting LLRs for forwarding for LLRcombining, a centralized controller determines which cooperating UEswill transmit LLRs corresponding to which RBs. This method reduces theD2D bandwidth used by i/N, compared to N cooperating UEs that eachforward all the LLRs to the target UE. However, this method utilizesadditional signaling bandwidth for communications with a centralizedcontroller. This method is similar to FSS scheduling used in cellularsystems. However, fairness is not considered.

Figure to illustrates flowchart 220 for a method of determining LLRs fortransmission in LLR combining by a cooperating UE. This method may beperformed by multiple cooperating UEs. Initially, in step 224, thecooperating UE determines the SNRs of the RBs by determining the ratiosof the signal powers to the noise powers. The average SNR per RB may becomputed using the EESM average of sampled SNRs at different tones ofthe RBs.

Then, in step 226, the cooperating UE transmits the SNRs of its RBs tothe centralized controller.

Next, in step 228, the cooperating UE receives a message from thecentralized controller. The message indicates which RBs that cooperatingUE should transmit the corresponding LLRs.

FIG. 11 illustrates flowchart 250 for a method of determining whichcooperating UEs will transmit LLRs corresponding to which RBs performedby a centralized controller. The centralized controller may be astand-alone device, a UE, or another device. Initially, in step 254, thecentralized controller receives the SNRs for the RBs from thecooperating UEs.

Then, in step 256, the centralized controller determines whichcooperating UEs will transmit LLRs corresponding to which RBs. In oneexample, the centralized controller determines that the cooperating UEwith the best SNR for a given RB will transmit the LLR corresponding tothat RB. Thus, only the cooperating UE with the best access link at eachRB transmits the corresponding LLR. In another example, the LLRcorresponding to the RB with the best SNR is only transmitted when theSNR that RB at the target UE is below a threshold. This further reducesthe D2D link resource usage.

In an additional example, in step 256, only the worst M of N RBs of thetarget UE are forwarded from the cooperating UE to the target UE. Thisexample further reduces the D2D link usage.

Finally, in step 258, the centralized controller sends messageindicating which LLRs are to be transmitted for all the cooperating UEs.These messages are transmitted to the cooperating UEs and the target UE.

FIG. 12 illustrates a block diagram of processing system 270 that may beused for implementing the devices and methods disclosed herein. Specificdevices may utilize all of the components shown, or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The processing system may comprise a processing unitequipped with one or more input devices, such as a microphone, mouse,touchscreen, keypad, keyboard, and the like. Also, processing system 270may be equipped with one or more output devices, such as a speaker, aprinter, a display, and the like. The processing unit may includecentral processing unit (CPU) 274, memory 276, mass storage device 278,video adapter 280, and I/O interface 288 connected to a bus.

The bus may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. CPU 274 may comprise any type of electronic dataprocessor. Memory 276 may comprise any type of system memory such asstatic random access memory (SRAM), dynamic random access memory (DRAM),synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof,or the like. In an embodiment, the memory may include ROM for use atboot-up, and DRAM for program and data storage for use while executingprograms.

Mass storage device 278 may comprise any type of storage deviceconfigured to store data, programs, and other information and to makethe data, programs, and other information accessible via the bus. Massstorage device 278 may comprise, for example, one or more of a solidstate drive, hard disk drive, a magnetic disk drive, an optical diskdrive, or the like.

Video adaptor 280 and 1/O interface 288 provide interfaces to coupleexternal input and output devices to the processing unit. Asillustrated, examples of input and output devices include the displaycoupled to the video adapter and the mouse/keyboard/printer coupled tothe I/O interface. Other devices may be coupled to the processing unit,and additional or fewer interface cards may be utilized. For example, aserial interface card (not pictured) may be used to provide a serialinterface for a printer.

The processing unit also includes one or more network interface 284,which may comprise wired links, such as an Ethernet cable or the like,and/or wireless links to access nodes or different networks. Networkinterface 284 allows the processing unit to communicate with remoteunits via the networks. For example, the network interface may providewireless communication via one or more transmitters/transmit antennasand one or more receivers/receive antennas. In an embodiment, theprocessing unit is coupled to a local-area network or a wide-areanetwork for data processing and communications with remote devices, suchas other processing units, the Internet, remote storage facilities, orthe like.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method comprising: receiving, by a first userequipment (UE), a message, for a second UE, transmitted over a pluralityof resource blocks (RBs) on behalf of a communications controller;determining a plurality of log-likelihood ratios (LLRs) in accordancewith the received plurality of RBs; and transmitting, a subset of thedetermined LLRs to the second UE.
 2. The method of claim 1, furthercomprising selecting a portion of LLRs of the plurality of LLRs, whereintransmitting the subset of the determined LLRs comprises transmittingthe selected portion of LLRs.
 3. The method of claim 1, wherein thefirst UE is a cooperating UE and the second UE is a target UE.
 4. Themethod of claim 1, wherein the step of determining includes identifyingat least one RB carrying a message destined for the second UE, anddetermining an LLR for each of the at least one identified RBs; andwherein the step of transmitting includes transmitting each of thedetermined LLRs to the second UE.
 5. The method of claim 1, furthercomprising demodulating data of the message to produce demodulated data,wherein determining the plurality of LLRs further comprises determiningthe plurality of LLRs in accordance with the demodulated data.
 6. Themethod of claim 5, wherein demodulating the data is phase-shift keying(PSK) demodulating the data, frequency-shift keying (FSK) demodulatingthe data, amplitude-shift keying (ASK) demodulating the data, orquadrature amplitude modulation (QAM) demodulating the data.
 7. Themethod of claim 1, further comprising determining a plurality ofsignal-to-noise ratios (SNRs) for the plurality of RBs, whereinselecting the subset of LLRs further comprises selecting the subset ofLLRs in accordance with the plurality of SNRs.
 8. The method of claim 7,wherein determining the plurality of SNRs further comprises determiningthe plurality of SNRs using an effective exponential SNR mapping (EESM)average of sampled SNRs at different tones of the plurality of RBs. 9.The method of claim 7, wherein selecting the subset of LLRs furthercomprises determining a subset of RBs of the plurality of RBs, whereinRBs of the subset of RBs have SNRs greater than a threshold, whereinselecting the subset of LLRs further comprises selecting the subset ofLLRs corresponding to the subset of RBs.
 10. The method of claim 9,further comprising receiving, by the first UE, the threshold.
 11. Themethod of claim 7, further comprising: transmitting, by the first UE toa central controller, the plurality of SNRs; and receiving, by the firstUE from the central controller, an indication message, wherein theindication message identifies the subset of LLRs.
 12. A methodcomprising: receiving, by a central controller from a first UE, a firstsignal-to-noise ratio (SNR) measurement corresponding to a resourceblock (RB); receiving, by the central controller from a second UE, asecond SNR measurement corresponding to the RB; and determining aselected UE to transmit a set of log-likelihood ratios (LLRs)corresponding to the RB in accordance with the first SNR measurement andthe second SNR measurement.
 13. The method of claim 12, wherein thefirst UE is a first cooperating UE and the second UE is a secondcooperating UE.
 14. The method of claim 12, further comprising promptingthe selected UE to transmit the set of LLRs corresponding to the RB. 15.The method of claim 14, wherein the selected UE is the first UE, andwherein prompting the selected UE to transmit the set of LLRscorresponding to the RB comprises transmitting, by the centralcontroller to the first UE, a message.
 16. The method of claim 12,further comprising transmitting, by the central controller to the firstUE, a message indicating the set of LLRs.
 17. The method of claim 12,further comprising transmitting, by the central controller to a targetUE, a message indicating the set of LLRs.
 18. The method of claim 12,wherein the central controller is a UE.
 19. The method of claim 12,wherein the central controller is a stand-alone device.
 20. A centralcontroller comprising: a processor; and a non-transitory computerreadable storage medium storing programming for execution by theprocessor, the programming including instructions to receive, from afirst UE, a first signal-to-noise ratio (SNR) measurement correspondingto a first resource block (RB), receive, by the central controller froma second cooperating UE, a second SNR measurement corresponding to thefirst RB, and determine a selected UE to transmit a set oflog-likelihood ratios (LLRs) corresponding to the first RB in accordancewith the first SNR measurement and the second SNR measurement.